Sampling

In Luker's Chapter 6 she discusses the process of sampling during research. I found this chapter to be pretty helpful when considering my proposal. Luker's correct in saying that you must sample when doing research because there is simply not enough time to collect every single piece of data. When I was first designing my basic proposal I thought I would be able to collect data from every single library in the Toronto Public Library System. Now, however, I am realizing that this would probably be beyond the scope of a thesis. I know I must take a sample and I guess the question remaining is: how large should my sample be? How many interviews are sufficient for this project? Another thing about my proposal is that I would like to gather data for two different samples: libraries in Toronto and highschool students in Toronto. I am in the process of reevaluating this. I want to ensure that I am not taking on too much.